Aluminum nitride (AlN) holds great promise as a semiconductor material for numerous applications, e.g., optoelectronic devices such as short-wavelength light-emitting diodes (LEDs) and lasers, dielectric layers in optical storage media, electronic substrates, and chip carriers where high thermal conductivity is essential, among many others. In principle, the properties of AlN may allow light emission at wavelengths down to around 200 nanometers (nm) to be achieved. Recent work has demonstrated that ultraviolet (UV) LEDs have superior performance when fabricated on low-defect AlN substrates prepared from bulk AlN single crystals. The use of AlN substrates is also expected to improve high-power radio-frequency (RF) devices made with nitride semiconductors due to the high thermal conductivity with low electrical conductivity. However, the commercial feasibility of AlN-based semiconductor devices is limited by the scarcity and high cost of large, low-defect single crystals of AlN.
To make large-diameter AlN substrates more readily available and cost-effective, and to make the devices built thereon commercially feasible, it is desirable to grow large-diameter (>25 mm) AlN bulk crystals at a high growth rate (>0.5 mm/hr) while preserving crystal quality. The most effective method of growing AlN bulk single crystals is the “sublimation-recondensation” method that involves sublimation of lower-quality (typically polycrystalline) AlN source material and recondensation of the resulting vapor to form the single-crystal AlN. U.S. Pat. No. 6,770,135 (the '135 patent), U.S. Pat. No. 7,638,346 (the '346 patent), and U.S. Pat. No. 7,776,153 (the '153 patent), the entire disclosures of which are incorporated by reference herein, describe various aspects of sublimation-recondensation growth of AlN, both seeded and unseeded. While these references recognize the benefits of a large axial (i.e., parallel to the primary growth direction) thermal gradient for optimizing material quality and growth rate of the growing AlN crystal, they utilize a growth apparatus designed to minimize the radial (i.e., perpendicular to the primary growth direction) thermal gradient. For example, axial thermal gradients may range from approximately 5° C./cm to approximately 100° C./cm, while radial thermal gradients are maintained at as negligible a level as possible. Likewise, other prior-art growth apparatuses utilize heavy insulation in order to minimize or eliminate the radial thermal gradient, as a minimized radial thermal gradient is expected to produce flat, high-quality crystals, particularly when efforts are made to grow crystals having large diameters. The radial gradient is typically minimized during conventional crystal growth in order to prevent formation of defects such as dislocations and low-angle grain boundaries. It is also minimized to make the surface of the growing crystal more flat, thus increasing the amount of usable material in the crystal (i.e., increasing the number of substrates that can be cut from the crystal for a given length of crystal).
FIG. 1 depicts an apparatus 100 utilized for the growth of AlN in accordance with the above-described prior art. As shown, a crucible 105 is positioned on top of a crucible stand 110 within a cylindrical susceptor 115. During the growth process, the susceptor 115 is translated within a heated zone created by surrounding heating coils (not shown), polycrystalline AlN source material 120 at the base 125 of the crucible sublimes at the elevated temperature, and the resulting vapor recondenses at the cooler tip 130 of the crucible due to the large axial thermal gradient between the base 125 and the tip 130, thus forming an AlN crystal 135. The apparatus 100 also features top axial shields 140 and bottom axial shields 145 designed and positioned to minimize the radial thermal gradient perpendicular to the growth direction 150 of AlN crystal 135. As shown, the tip 130 of the crucible 105 is cooler than the base 125 at least in part because apparatus 100 has fewer top axial shields 140 than bottom axial shields 145, allowing more heat to escape in the region of tip 130 and generating the desired axial thermal gradient. The top axial shields 140 may have centered holes therewithin to facilitate measurement of the temperature at tip 130 by a pyrometer 155. The centered hole diameter is minimized to reduce the heat flow but sufficient to form a practical optical path for the temperature sampling by the pyrometer 155. Additional pyrometers 160, 165 may also be utilized to measure temperatures at other regions of apparatus 100.
As mentioned above, the ability to grow AlN single crystals at high growth rates would spur additional commercial adoption of the technology. While increasing the growth rate of AlN crystals is theoretically possible by increasing the Al supersaturation using larger axial thermal gradients, increases in the Al supersaturation may result in deterioration of the material quality of the crystal, or even in polycrystalline, rather than single-crystal, growth. Furthermore, the minimization or elimination of radial thermal gradients during AlN crystal growth unexpectedly tends to deleteriously impact the quality of the AlN crystal, particularly when attempts are made to grow large (e.g., >25 mm diameter) crystals at reasonable growth rates (e.g., >0.5 mm/hr). Thus, a need exists for systems and techniques enabling growth of such large AlN crystals at high growth rates while still preserving high material quality of the AlN crystal.